One of the basic properties of shapes and objects is symmetry. The human body shows symmetry along an axis oriented from top to bottom.
Medical images originating from image acquisition modalities such as computed projection radiography show symmetry to a large extent for examination types of body parts which are essentially symmetric. Examples of such examination types are e.g. frontal skull, frontal spine, frontal thorax, frontal abdomen, frontal pelvis.
In U.S. Pat. No. 5,533,143 a method is disclosed for determining whether an image is symmetric or asymmetric. Image processing conditions are based on the judgment that the image is a frontal (symmetric) object image or a lateral (asymmetric) image.
The disclosed method is based on the assumption that the hypothetical symmetry axis is always fixed and coincides with a vertical center line passing through the center of the image. The method does not discriminate between vertical or horizontal symmetry.
By studying a large mixture of all symmetric examination types, it is observed that (1) the symmetry line in most cases does not coincide with the center line of the image, nor is its angle with respect to a perfectly vertical (or perfectly horizontal) line equal to zero and (2) horizontal symmetry occurs equally well as vertical symmetry.
The above-mentioned observations limit the accuracy and applicability of the prior art method disclosed in the above-mentioned patent.
In U.S. Pat. No. 4,903,310 and U.S. Pat. No. 4,951,201 various histogram based methods are proposed to discriminate between a frontal and a lateral body posture.
It is however not possible to decide by applying these methods whether the object is symmetrical and whether the object is positioned horizontally or vertically.
The above methods are thus not adequate.
Experience has learnt that radiologists prefer to examine radiographic images in standard viewing conditions, i.e. with the image displayed so that a symmetry axis in the image is substantially horizontally or vertically oriented.